Spongy Saturn moon is crater 'museum'

Saturn's bizarre moon Hyperion looks for all the world like a giant sea sponge. Now, astronomers think they know why - it is so porous that space rocks hitting it simply crunch into its airy body without sending up much debris, creating deep, long-lived craters.

Hyperion is one of the solar system's oddest objects. The heavily cratered moon has a very irregular shape, measuring 360 kilometres long and 250 km across. And it rotates chaotically, tumbling unpredictably due to the gravitational influence of Saturn and its large moons.

"Hyperion is utterly unique among all the little bodies we've looked at in the solar system," says Dale Cruikshank of NASA's Ames Research Center in Moffett Field, California, US.

This view of Hyperion was only strengthened when the Cassini spacecraft observed it at close range during four flybys in 2005 and 2006. The spacecraft found that it has many more craters than are typically found on Saturn's other moons and that the craters themselves are deeper and less eroded than usual. Some also have a mysterious dark material covering their floors.

"When the images came in a couple of years ago, they were head-scratchers," says Peter Thomas of Cornell University in Ithaca, New York, US.

Now, he and colleagues have made detailed models of the moon's shape and surface using Cassini data and they say the moon's sponge-like appearance is due to its porosity. The spacecraft's measurements suggest the moon is 42% empty space, with the rest being mostly water ice.

Dirty snowball

In laboratory experiments, projectiles fired at such porous objects tend to produce craters without throwing up a lot of ejecta, and the researchers think the same process occurs on Hyperion. "The craters stay a little fresher and deep as they accumulate, so the surface looks more like a sponge rather than smoothing everything out," Thomas told New Scientist.

Another team, led by Cruikshank, has analysed the composition of the moon's surface using Cassini's Ultraviolet Imaging Spectrometer (UVIS) and its Visible-Infrared Mapping Spectrometer (VIMS).

That team finds that the moon, which reflects just 30% of the light that falls on it, is made of water ice that has been dusted with reddish black organic material, though its exact composition is unclear.

That same material appears to be what is concentrated at the bottoms of some of the moon's craters. "But where it came from, how it got there, and how it got concentrated - those are some of the big remaining puzzles," Cruikshank told New Scientist. Similar material is found in relatively high amounts on two of Saturn's other moons - Phoebe and Iapetus.

Blow out

The researchers also found carbon dioxide on the moon, but not in the pure form seen on many other bodies in the solar system - pure dry ice would have evaporated in about a million years on Hyperion. Instead, the moon's CO2 appears to be attached to another molecule, possibly water, allowing it to remain on the moon for much longer - perhaps for the moon's lifetime of about 4.6 billion years.

"The CO2 is, I think, a critical component of this object and may tell us something about how it came to be, how it came to get so porous," says Cruikshank.

Researchers have previously speculated that Hyperion either coalesced from the rubble of a collision between space rocks or that it formed very slowly, like snowflakes that stuck together gently over time. But Cruishank says another process may explain the moon's porosity.

"Maybe when it originally formed and solidified, it contained vast amounts of CO2 and other volatiles, and that stuff is largely gone now, evaporated out of the middle," he says. "If it exploded out, some of these craters would be blow-out craters rather than impact craters."

Cassini: Mission to Saturn - Learn more in our continually updated special report.

Journal references: Nature (vol 448, p 50 and 54)

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Saturn's moon Hyperion resembles a sea sponge in this portrait, which was made by combining infrared, green and ultraviolet images taken by Cassini when it flew by the moon in September 2005 (Image: NASA/JPL/Space Science Institute)